Method of restricting a solid propellant



United States 3,107,573 METHGD {1F RESTRICTENG A SOLID PROPELLANT Thisinvention relates to solid propellants provided with a flame restrictioncoating on a portion of the surface thereof.

Solid propellants for use in rocket motors or gas generators must burnin a controlled manner. This control is obtained by forcing thecombustion to be limited to selected portion(s) of the surface of thesolid propellant grain. For example, a rod-like grain which is desiredto burn in cigarette fashion must be prevented from burning on thecylindrical surface thereof. Regardless of how tightly the grain may befitted into a rocket motor or a gas generator combustion chamber, hotgases tend to leak between the wall and the surface of the grain andcause combustion where it is not wanted. Solid propellant grains areforced to burn in a desired manner by applying to selected portion(s) ofthe surface a flame restriction coating commonly called a restrictor.The restrictor is a material which is either relatively inert or burnsso much more slowly than'the propellant material itself that thepropellant beneath the restrictor does not burn until the flame reachesit.

Many materials have been used as restrictors. One of the earliestmaterials and one still widely used is cellulose acetate sheet. Forexample, in a rod that is desired to burn cigarette fashion a sheet ofcellulose acetate is glued to the cylindrical surface. This proceduresounds simple but the job of adhering the sheet to the surface has beenfound to be difiicult and frequently unsatisfactory. Also, where thereare irregular surfaces to be restricted, it is diflicult to shape andadhere cellulose acetate sheet to these irregular surfaces. On the otherhand, cellulose acetate is relatively incombustible and reasonably cheapand can be softened by many solvents and plasticizers so that it is avery suitable restricting material when cellulose heet can be applied tothe surface of the grain.

It has been discovered that all types of solid propellants can be verysuccessfully restricted with cellulose acetate restrictor coating whenthe coating is formed from cellulose acetate fibers adhered to thesurface of the solid propellant body which is desired to be restricted.

The term fiber as used herein means a thread like material capable ofbeing spun, woven, knitted, or felted. The restrictor coating may beformed from cellulose acetate fibers which are in the form of wovenfabric, knit fabric, felt fabric, or formed into non-woven cloth whereina binder holds the fibers in place. All types of woven and knit fabricswhich are available from the textile industry are suitable for use inthe invention.

The cellulose acetate fiber may be derived from any cellulose acetatewhich can be formed into thread like structures. By the use of specialsolvents, fibers can be formed from material which is essentiallycellulose triacetate. In general, the cellulose acetate fibers will beformed from material having a combined acetic acid value of 53-60% Thesolid propellant grain consists essentially of a body formed from theparticular solid propellant and adhered celulose fiber restrictorcoating. The solid propellant body may be made up of any of thematerials used for this purpose. These propellants may be double basepowder, ammonium nitrate, ammonium perchlorate, etc. The ammoniumnitrate and ammonium perchlorate propellants comprise the oxidizer and acombustible organic atent "too material binder therefor. These bindersmay be thermoplastic materials which are a mixture of organic polymerand a plasticizer therefor. For example, cellulose acetate, celluloseacetate butylate, polyvinyl chloride, polyvinyl acetate, are suitablepolymers with ammonium nitrate oxidizer. Numerous suitable plasticizersare known for these materials, such as triethyl citrate, dinitrodiphenyl oxide, dinitro toluene, and triacetin. Rubber-like materials,such as Thiokol rubber, GRS rubber, and the like may be used as binders.

The coating of cellulose acetate fiber must be adhered to the surface tobe restricted. This is preferably done by solvent-softening the fabric.The softening may be accomplished by a material which is capable ofdissolving the particular cellulose acetate or merely softening thefibers and causing stickiness; or a plasticizer type solvent may be usedto accomplish the softening of the cellulose acetate fibers. It isdesirable to use as the solvent-softener a material which will alsosoften appreciably the surface of the solid propellant body to which thefabric is to be adhered. Examples of suitable solventsofteners are:methyl acetate, acetone, methyl ethyl ketone, commercial ethyl acetate,99% ethyl acetate, methyl Cellosolve acetate, methyl Cellosolve,Cellosolve acetate, nitrornethane ethyl alcohol, nitromethane,nitropropane ethyl alcohol, nitropropane methanol, ethyl lactate,diacetone alcohol, methylene chloride ethyl alcohol, ethylene dichlorideethyl alcohol. Illustrative of suitable plasticizers are triethylcitrate, acetyl triethyl citrate, triacetin, nitrotoluene, etc.

The amount of solvent-softener used will be dependent upon theparticular material, the amount of restrictor coating needed, and thetype of configuration to be restricted. In general, sufiicientsolvent-softener is used to adhere the fabric to the surface to permitfit of the fabric to surface irregularities during the enplacingoperation.

The fabric may be applied to the surface by wetting the fabric with theplasticizer or solvent and then applying the wet fabric to the surface.In the case of a rod or similar material whose cylindrical surface is tobe restricted, the fabric may be wrapped around the surface for theparticular number of layers needed to provide the desired amount ofrestriction. Instead of wetting the fabric prior to applying it to thesurface, the fabric may be positioned on the surface and then wet withthe plasticizer or solvent. When end burning rods are desired, it ispreferred to wet the fabric prior to positioning as this avoids wrinkleswhen several layers of fabric are needed for the particular coating.When a low-boiling solvent, such as acetone or methyl acetate is used,the solvent may be permitted to evaporate at ordinary temperatures or atsomewhat elevated temperatures, dependent upon the boiling point of thesolvent and the facilities available. In the case of solvent softenerswhich are more in the nature of plasticizer, the finished grain isobtained by heating the body and wet fabric affixed thereto at atemperature suitable for setting the fabric. When a material such astriethyl citrate or triacetin is the solventsoftener, temperatures onthe order of ISO-250 F. may be used. Sufiicient time is permitted at theparticular temperature for the removal of the solvent for setting of theplasticizer. The particular time will be dependent upon the temperature,the solvent, or the plasticizer and the thickness of the restrictorcoating.

The dried grain has a tough, resilient covering at the portion where thecellulose acetate fabric has been adhered. The coating performs as wellas cellulose acetate sheets applied by the more usual methods. It hasbeen noted that thicker coatings are permissible with the celluloseacetate fabric method of applying the restrictor coating. It has alsobeen observed that the presence of the bubbles beneath the fabricrestrictor or between layers of fabric does not impair theflame-restriction quality of the coating as it does so frequently incellulose acetatesheet restrictors. V

The cellulose acetate fabric restrictor has been utilized with ammoniumnitrate propellants in all the manners in which other restrictors havebeen applied. For example, as coatings on rods to provide cigaretteburning; as annular discs aifixed to the end of tubular grains whichprovide burning on the cylindrical surfaces inward toward and outwardfrom the center of the grain; also as annular discs applied to the endsof tubes and as cylindrical surface coatings providing burning from theinside toward the outer surface of the grain. It is preferred that therestricted grain be positioned in the rocket motor or generatorcombustion chamber so that the restrictor surface is close fittingagainst the wall of the motor or chamber. Where insulation is desired toprotect the'metal of the chamber, appropriate insulators may be appliedto the restricted surface or the grain may be inserted into an asbestostube for use as a cigarette burning grain.

An illustrative ammonium nitrate propellant which has been restrictedwith the cellulose acetate fabric by the method of the invention is:ammonium nitrate oxidizer, 62%; binder made up of cellulose acetate(54-56% combined acetic acid), 12%; acetyl triethyl citrate, 9%;dinitrophenoxyethanol, 6%; and bis(dinitrophenoxy)ethane, 3%; inaddition the propellant contained carbon black, 4%; toluene diamine, 1%;and catalyst, 3%. Cigarette burning grains ranging in diameter from oneinch to five inches were prepared by applying commercially purchasedacetate rayon cloth to the outside cylindrical surface. Inside-outsideburnings were prepared by restricting the annular end surface of a fiveinch diameter hole.

These test grains were prepared by the plasticizer method with atriethyl citrate or triacetin to soak the cloth before applying thecloth to the surface to be restricted. In multi-layer applications, thewet cloth was then covered with a damp cloth to seal off the wet cloth.For convenience, the burners were inserted into an asbestos sleeve tosupport the grain during the time that the grain was maintained at 170F. for about 12 hours to set the restrictor coating. Other grains wereprepared by wrapping dry cloth about the grain and then wetting withacetone or methyl acetate. The wet grains were permitted to remainexposed at room temperature of 1. A method of restricting a solidpropellant body which method consists essentially of (a) applying asolvent-softened fabric formed from cellulose acetate fiber to aselected portion of the surface of a solid propellant body, in thethickness desired to accomplish flame-restriction, and (b) heating saidcoated body to a temperature and for a time needed to adhere said fabricto said surface. 7

2. The. method of claim 1 wherein said solvent is acetone.

3. The method of claim 1 wherein said solvent is methyl acetate.

4.'The method of claim 1 wherein said solvent is triethyl citrate.

5. The method of claim 4 wherein said temperature is about 250 F. e

6. The method of claim 1 wherein said fabric is a woven fabric.

7. The method of claim 1 wherein said fabric is a knit fabric.

8. The method of claim 1 wherein said fabric is a felt fabric.

9. The method of claim 1 wherein said body comprises ammonium nitrateoxidizer and a combustible organic material binder therefor.

References Cited in the file of this patent UNITED STATES PATENTSCampbell et al Nov. 1, 1960

1. A METHOD OF RESTRICTING A SOLID PROPELLANT BODY WHICH METHOD CONSISTSESSENTIALLY OF (A) APPLYING A SOLVENT-SOFTENED FABRIC FORMED FROMCELLULOSE ACETATE FIBER TO A SELECTED PORTION OF THE SURFACE OF A SOLIDPROPELLANT BODY, IN THE THICKNESS DESIRED TO ACCOMPLISHFLAME-RESTRICTION, AND (B) HEATING SAID COATED BY TO A TEMPERATURE ANDFOR A TIME NEEDED TO ADHERE SAID FABRIC TO SAID SURFACE.